In recent years, the small hermetically sealed rechargeable electrochemical cell has seen a number of advances in, e.g., its structure, its assembly and in the materials used in its manufacture. The basic construction of such a cell is shown in my prior patent, U.S. Pat. No. 3,503,806, assigned to the assignee of the present invention. That cell includes the familiar cylindrical casing and a spirally wound electrode assembly in which the positive and negative plates are longitudinally offset from one another, as well as from a separator, to provide an extended plate edge at one end of the electrode coil and another extended plate edge at the opposite end of the coil. The structure is provided with a conductive interconnecting strap which electrically connects the edges of the respective electrode coils to the cell terminals. This strap is attached at a number of contact points by percussion welding techniques. Some recent improvements to this cell structure have been suggested. In U.S. Pat. No. 4,029,856, it is disclosed to form the terminal strap in a rectangular shape having leg portions welded to the edges of the spirally wound electrode plate and having a central raised portion which is attached to the underside of the cell closure cap.
While the electrochemical cells of the foregoing type have proven to be very effective for low capacity service, i.e., up to about 10 ampere-hours, they are unfortunately not conductive to high capacity operations, due primarily to the structure and design of the terminal connections. These connections somewhat impede current flow and may generate undue amounts of heat at high current levels which can result in cell degradation or, in the case of malfunction of an emergency vent, dangerous pressure build-up. As is known in the art, generation of excessive heat and pressure in an electrochemical cell can present serious dangers to users and can cause damage to the cell due to cell degradation and hastened decomposition. This limitation of the aforementioned cells to carry high currents has discouraged their use in high current applications.
The solution of this problem has been to use an entirely different cell construction for high currents. Such high current cells are usually of the vented type wherein the cell is vented to the atmosphere in order to release gases generated internally and wherein liquid electrolyte floods the cell casing. Rather than using a spirally wound electrode assembly, the vented cells usually employ flat electrode plates having large plate-like terminal straps which are welded to the electrodes and are brought upwardly toward the top of the cell where they are affixed to a large terminal lug.
The flat plate vented cell succeeds for high current applications where the coiled electrode cell has not because, in part, the flat plates are well adapted to affixing heavy interconnecting conductive terminal straps between the electrodes and the terminal lug. Known sealed cell constructions are not adapted to low impedance connections between electrodes and terminals.
Although the flat plate vented cells provide high current delivery, the plate-to-terminal connections are costly to manufacture and their assembly is difficult since much of the fabrication must be done by hand. Moreover, vented cells ordinarily must be used in the upright position to ensure that the liquid electrolyte does not block the vent. Another restriction resides in the fact that flat plate cells are housed in rectangular impact-resistant containers and when several such cells are interconnected in a common case to form a battery, air cooling of the cells is inhibited unless the cells are spaced apart. Increasing cell spacing necessitates increasing the overall dimensions of the battery case which, under certain circumstances such as for uses in aircraft, can be quite unacceptable.
Accordingly, it is an object of the present invention to provide a sealed electrochemical cell of the spiral wound type having high current capacity.
Other objects of the invention are to provide high capacity sealed electrochemical cells of the spiral wound electrode type which have reduced internal impedance, improved means for dissipating internally generated heat during high current discharge, and improved means of assembly.
These and other objects will become apparent from the following summary and description of the invention taken in conjunction with the accompanying drawings.